x264 only produces High 4:4:4 Predictive Profile H.264 when using its lossless mode.
Lossless H.264 is enabled when using a constant QP of 0; in HandBrake's 8-bit build of x264, a Constant Quality (CRF) of 0 is equivalent.

Very few decoders and/or video editing software support High 4:4:4 Predictive Profile H.264.

It's only suitable for lossless intermediate encodes - i.e. if you're encoding content for watching it, it's not what you need.
A constant RF of 19 is generally considered transparent to the source.